1. Field of the Invention
This invention relates to an apparatus and a method for grinding particulate material such as toner or the like.
2. Related Art
Particulate material such as toner is generally ground in a fluidized bed jet mill grinder. A fluidized bed jet mill grinder operates by accelerating the particles in the particulate material using jets of air. The particles then impact either on a surface, or other particles, causing them to fragment. By providing fluidized bed jet mill grinder in an enclosed chamber, the particles undergo repeated impact until they are substantially reduced in size. This ground material may then be removed from the grinding chamber.
U.S. Pat. No. 5,133,504 places an impact target at the center of the grinding chamber. The target is configured such that the particles are accelerated toward the target by one of several jets of air, causing the particles to impact on the target surface. While this causes the particles to fragment, resulting in smaller particles, there is generally heavy wear of the target. This in turn causes the target to become less efficient, causing a reduction in the overall efficiency of the grinding chamber. As a result, the impact targets needs to be regularly replaced.
U.S. Pat. No. 5,562,253 discloses an impact target which includes a number of apertures. Each aperture is aligned with a jet of air, such that the jet of air passes through the aperture to the center of the target. Accordingly, any particles caught up in the edge of the jet of air will impact on the target, while any particles in the center of the jet of air will pass through the aperture and impact with other particles from other jets of air in the center of the target. The 253 patent also discloses a flat plate impact target having a single aperture. Each plate is aligned with an associated jet of air. The jet of air again passes through the aperture towards the center of the grinding chamber.